Salmān al-Fārisī's Trench

And the Messenger of Allah (peace be upon him) is following the news; again, he had his men spread out all over, gathering intelligence for him. So the news reached the Messenger of Allah (peace be upon him), prior to them advancing towards Madīnah, that a huge army was coming his way. The Messenger of Allah (peace be upon him) holds a shūrā, and Salmān al-Fārisī (may Allah be pleased with him) has a suggestion. He said: Yā Rasūl Allāh, innā idh kunnā bi-arḍi Fāris wa-takhawwafnā al-khayl, khandaqnā ʿalaynā. Fa-hal laka yā Rasūl Allāh an tukhandiq? He said, "O Messenger of Allah, in our land, in Persia, if we feared the cavalry, we would dig a trench or a moat. O Messenger of Allah, why don't we dig a moat, a trench?" The Messenger of Allah (peace be upon him) and the Muslims liked the idea, and the project started.

Now, it would have been impossible for them to dig this moat all around Madīnah, because they just didn't have the manpower for that, so they had to choose the locations which were vulnerable. Now, that location was north of Madīnah, which would sound strange, because Makkah is to the south. But the thing is that Madīnah had a natural protection from the east and west, and that was what is called al-Ḥarrah. These are volcanic tracts, tracts of volcanic rock, very sharp, very edgy, with boulders all over, so it is difficult for an army to march through. And then from the south there are the dwellings and farmland, which also makes it difficult to penetrate. So the danger is from the north, and that is where they dug the trench.